Use your developer tools, in most modern browsers you can select an element on the page and see what CSS rules apply to the element, you'll also be able to see exactly what line of the style-sheet the code can be found. F12 seems to be the standard way of accessing developer tools.
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JasperDec 20 '12 at 19:55

So there's no selector to select all the texts at once (including labels, buttons, ...)?
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DistwoDec 20 '12 at 18:25

@Distwo - html { ... } would set the font for the entire page, but you need to have a selector as specific or more specific than the jQuery Mobile selector to override its value (otherwise, the jQuery Mobile selector would take precedence).
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Justin NiessnerDec 20 '12 at 18:27

1

For many reasons, Is a very bad idea to change JQuery Mobile CSS. Just create your own style and override the JQM's font.
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mandrilFeb 16 at 21:33

Solution

Final notes

If you want to find more about how to customize jQuery Mobile page and widgets then take a look at this article. It comes with a lot of working examples, including why is !important necessary for jQuery Mobile.